A modern pulp plant is composed of several departments or processing stations which include, for example, digestion, screening, washing and bleaching departments. The process flow or so-called "fiber line" of a pulp plant is composed of these processing stations, each including its own characteristic process steps or stages for treating the pump or fiber suspension. These individual departments comprising the fiber line are usually located in a three-story building with an individual story height of about 6 m or more.
Gas is present in pulp suspensions mainly in three forms, namely, in the form of small bubbles, dissolved or chemical bound gas.
The chemically bound gas or dissolved gas seldom causes problems in the pulp and papermaking processes but can cause problems if conditions are changed and bubbles start to form.
Gas bubbles in the fiber suspension can be present as free bubbles in the liquid between the fibers or as bound bubbles attached to fibers. Both bound and free bubbles cause problems in the papermaking processes. Free bubbles cause special problems in the pulp and papermaking processes when they are present in too great an amount. The problems include foam problems, instability of the processes, decreased deaerating, and the like.
The method of the present invention relates to the separation and removal of most of the free air bubbles so that the problems caused by an excess amount of free air bubbles are eliminated.
Total gas removal is generally accomplished by another type of gas removal, so-called mechanical gas separation. With this method, all of the free and bound gas bubbles are removed. Also part of the dissolved gas is removed. This type of gas removal is performed immediately in front of the paper machine forming section to avoid pinholes and other problems on the forming wire. This method, which is described by K. D. Kurz, Tappi Engineering Conference, Sept. 19-21, (1978), is expensive and creates large amounts of foam when the fiber suspension is ejected with high speed onto a metal surface in a vacuum tank.
The traditional degassing assemblies in the pulp and paper industry are remarkably space demanding and hence costly, and the separated gas occurs in large volumes, from which reclaiming and conveying thereof is difficult. The most usual degassing equipment is a tank having a large diameter in which the gas in a gas contained liquid is permitted to rise to the liquid surface of the tank for removal. In order to be certain that a sufficient time period for degassing is given, the diameter of such a tank in large pulp plants can be 10-20 m and the height 5-6 m. It will thus be apparent to persons of ordinary skill that investment costs for a degassing tank of this kind are high and the reclaiming of gas therefrom is difficult.
Despite the fact that gas or air is usually present at certain process stages in the manufacture of pulp and paper, the presence of gas causes considerable disadvantages in both, the final product as well as in the treatment of the pulp or fiber suspension. As the most pronounced disadvantages caused by the entrainment of gas or air in the fiber suspension as they relate to the above-mentioned pulp and paper processes may be mentioned:
problems caused by the generation of foam in the above-referred to processes, PA0 increasing capillarity and needle perforation, PA0 instability of the fiber suspension in conduits, valves, screens, and the like, PA0 increased pump cavitation, PA0 dewatering problems, PA0 fiber flocculation, and PA0 decreasing formability when the stock is deposited on the wire of a paper machine. PA0 (a) transferring the gas containing aqueous fiber suspension at a consistency to a treatment station at a level; PA0 (b) treating the fiber suspension at said level by removing a portion of the water from the aqueous fiber suspension so as to generate aqueous filtrate; and PA0 (c) simultaneously pumping and degassing the filtrate at said level.
The fiber line in a paper manufacturing plant is usually designed so that the different processing stations are arranged and housed in one building, that is, generally inside a three-story building wherein, as mentioned, the individual story height is about 6 m or more. According to the present invention the conventional process stations are modified to such an extent so as to dispense with the traditional multi-story pulp plant. Instead, the present invention permits the use of a single-story construction without substantially changing the dimensions or design parameters of the respective process stations located therein.
If the process stations of a fiber line were to be altered to be accommodated in a single-story plant, major parts of the equipment used would also require substantial redesigning. This new equipment is often more complex and considerably more expensive than the traditionally utilized equipment. However, investment costs for an entire fiber line of a pulp plant will decrease about 20-40% if the building housing the fiber line can be constructed lower and smaller. This is due to the fact that in addition to the building, all conduits, cable bundles, air conditioning, instrumentation and the like, are reduced in size relative to the building.
FIG. 1 illustrates schematically a fiber line of a traditional pulp plant including a digestion, screening, washing and bleaching station, whereby the individual process stations in the Figure have been separated from each other for sake of clarity by dotted lines.
FIG. 1 shows that part of the digester house which, as a part of the fiber line, is placed inside the building. The digester itself (not shown) is usually placed out of doors. A chip silo 1 is located on the roof 10 of the building. From the chip silo 1, chips drop through a chip gage 3 and a low pressure distributor 4 into a presteaming vessel 2. The presteaming vessel 2 is located on a third level 9. From the presteaming vessel 2, the chips fall via contaminant separation device 5 into a high pressure feeder 6 on the ground level 7, wherefrom chips flow into a digester generally located outside the building. As can be seen from the Figure, the chip distribution system of the digestion station requires a three-story building although an intermediate level 8, usually existing in the building, is not being utilized in this stage.
After the digestion house, at the screening stage, a pump 21 at the ground level 7 feeds pulp to be screened and freed from impurities such as sand, grit and metallic particles into a screen 22 at the intermediate level 8, wherefrom the pulp flows into a filter 23, usually a vacuum filter, at the top level 9. From the filter vat 28 filtrate flows through a suction or barometric leg 26 into a filtrate tank 24 at the ground level 7. The vacuum filter is installed at an elevated level so that the filtrate flowing down the suction leg 26 to the filtrate tank 24 creates the necessary vacuum within the filter 23. The distance between the bottom of vat 28 and the surface of the filtrate in filtrate tank 24 is usually at least about 10 m.
In the washing section subsequent to the screening stage, washing of the pulp is carried out by transferring the cooked pulp from the digester to remove the black liquor therefrom and to recover the spent cooking chemicals. Washing the pulp is generally achieved by suction or pressure filters such as a rotary vacuum or pressure cylinder. Normally, 2 to 4 of these washers are operated in series with counter-current flow of washing liquid. Foam which has a negative effect on washing efficiency is thereby generated depending on the type of pulp. Generally, the washer comprises a hollow pipe axis through which the filtrate is permitted to flow into a suction leg 36, usually about 10-12 m long, to be discharged into a filtrate tank 34. The filtrate tank 34 is of sufficient size to permit the entrained air to separate from the filtrate before the filtrate is pumped back to the repulping stage or the preceding washer showers. After the last washer, the pulp is transferred to a storage tank at high consistency. Depending on the type of the washer, a certain structural height is always required. Usually, placement of a washer in the plant is similar to that of the screening stage, i.e. pulp is fed from the ground level by means of a pump to a filter 33 at the top level 9 of the building, wherefrom filtrate flows into a filtrate tank 24 at the ground level 7.
Traditionally, after the washing stage, the bleaching of pulp is carried out in the bleach plant again by means of a three-story arrangement. A pump 41 discharges pulp from a bleaching tower (not shown) for distribution via feed pipe 48 to usually a plurality of suction washers 42, wherein soluble chemicals of the bleaching operation are removed by repeated dilution and thickening or by displacement. After washing, the pulp is transferred through a drop leg 40 from top level 9 to a high consistency pump 43 at the ground level 7 from where the pulp is pumped via a chemical mixer 44 into another bleaching tower to continue the bleaching reaction. The bleaching towers, usually located outside the building, are designed to ensure the efficiency of the bleaching reaction. Inside the building are placed usually only the apparatuses illustrated in the Figure, whereby the washer 42 is placed at the top level 9 and the suction required for separating the filtrate from the pulp is generated by means of a suction leg 45 which extends from the washer 42 on the top level 9 down to a tank 46 at the ground level 7.
One object of the present invention is to adapt the processes of a fiber line and to thereby simplify the construction of a pulp plant so that the above-described fiber line process stages or stations can be accommodated inside a one-story building. A further object of the present invention is to enable an essentially gas-free distribution of fiber and pulp suspensions from one process station to another without the need for customary degassing devices, preceding a pump, such as, for example, deaerating chambers, cyclones, drop conduits, suction legs and the like, wherein differences in height, the formation of droplets, variations in speed and vacuum are utilized for carrying out the degasification operations, and which for the greater part thereof require a considerable structural height.